--check-updates should work with file:// dependencies

[facundobatista]

When you call fades with a file dependency (like, fades -d file:///tmp/path/to/project) it will not re-create the venv even if the project changes.

We could store the directory ctime in the fades metadata file, and use it to check updates and re-create the venv if changed.

This would be an example of use sequence:

• fades -d file:///tmp/path/to/project: a venv is created first time
• fades -d file:///tmp/path/to/project: the venv is reused
• the project mentioned in the dependency is changed
• fades -d file:///tmp/path/to/project: the venv is just reused!
• fades --check-updates -d file:///tmp/path/to/project: the project's dir ctime changed! a new venv is created
• fades --check-updates -d file:///tmp/path/to/project: the project's dir ctime didn't change, the venv is reused
• the project mentioned in the dependency is changed again
• fades --check-updates -d file:///tmp/path/to/project: the project's dir ctime changed! a new venv is created